[R] Mixed-effects models and model selection

[Fariba]

Hi,

I am looking at the effect of allelochemicals produced by two freshwater
macrophyte species on two different algal species at different days. I am
comparing the effect of each macrophyte on each algae at each day. I
received help from someone doing the LMEM (Linear mixed-effect models) and
he told me to do ANOVA to analyse the LMEM. However, I received these
feedback from my examinor;

1. An Analysis of Variance (ANOVA) was used to analyse the LMEM. 
Comment; Not clear why you used ANOVA here. Was it to compare models?

2.The effects of TREATMENT and TIME and their interaction were all
significant (Table 5).  Because of the significant interaction, the analysis
was split by TIME. 
Comment; Given that you have interactions, you should do a model selection
to show whether the interaction model is  in fact more parsimonious. 

Can someone explain these and tell me how and when should I do model
selection? 

Thanks,
F



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